In recent years, improvements of performance, such as energy saving, enhancement of printing speed, and high densification of print density, have been required for heat-sensitive recording using a thermal head, and producing a high-quality image with a high sensitivity even at a low thermal energy have been required for a heat-sensitive recording body for use in the heat-sensitive printing.
On the other hand, the applications of the heat-sensitive recording body are widespread, and recently, the heat-sensitive recording body has been used in a wide range of applications, such as labels, tickets, and receipts, including applications of early stages, such as facsimiles, terminal printers for computers, and recorders for measurement. As the range of use applications expands, high-speed recording performance, namely, high-quality image recording with a high sensitivity has been required for physical distribution labels, food labels, handy terminals for meter reading, tickets, and the like. For example, making it possible to perform good recording in which occurrence of blur is reduced, and difference in density between a printed portion and a non-printed portion is clear at a high speed is required for the heat-sensitive recording body. Further, information that is already known is generally printed in advance on the heat-sensitive recording face with offset printing, and in this case, offset printability is required for the surface of the heat-sensitive recording paper.
Conventionally used methods for enhancing the sensitivity of a heat-sensitive recording body are broadly classified as follows.    (1) Enhancement by using a chemical or chemicals such as a leuco dye, a developer, and a sensitizer (collectively referred to as color formers).    (2) Enhancement by devising a layer configuration of the heat-sensitive recording body.
The present invention relates to (2) the layer configuration of the heat-sensitive recording body and relates to an improvement of an intermediate layer to be provided between a support and a heat-sensitive color forming layer.
The rolls of the heat-sensitive recording body in the enhancement of sensitivity brought about by providing the intermediate layer are as follows.    (1) Smoothness of the surface of the heat-sensitive recording body is maintained to increase the contact area with a thermal recording head (thermal head), thereby improving the heat transmission to the heat-sensitive color forming layer.    (2) The heat-sensitive recording body exhibits an effect that the heat transmitted to the heat-sensitive color forming layer is retained in the heat-sensitive color forming layer, so that the heat is not dissipated on the base paper side to utilize the heat effectively for color-forming reaction, a so-called heat insulating effect.
For example, Patent Literature 1 describes that the sensitivity is enhanced by providing an intermediate layer mainly comprising a hollow particle between a support and a heat-sensitive color forming layer. Besides, it is known that a bulky inorganic pigment such as silica or calcined clay having an oil absorption of 80 ml/100 g or more (JIS-K5101) is used as a material for forming an intermediate layer, but the effect of enhancing the sensitivity is inferior to that obtained by the intermediate layer formed using a hollow particle. As the hollow particle, an inorganic substance and an organic substance are known. However, a hollow particle made of the inorganic substance has a large particle diameter and therefore cannot be put into practical use for an intermediate layer of heat-sensitive recording paper.
The hollow particle made of the organic substance is classified into two as described below, and it is known that both a foamed hollow particle and a non-foamed hollow particle can be put into practical use for an intermediate layer of heat-sensitive recording paper.    (1) A foamed hollow particle (also referred to as thermally expandable hollow particle) obtained by encapsulating a volatile substance such as isobutane with a thermoplastic polymer containing vinylidene chloride and then expanding the resultant encapsulated isobutane by heating to make a hollow body, the foamed hollow particle described in Patent Literature 2 or Patent Literature 3.    (2) A non-foamed hollow particle produced by an alkali swelling method, the non-foamed hollow particle described in Patent Literature 4 or 5.